High-frequency signal line

ABSTRACT

A high-frequency signal line includes a dielectric body including a first dielectric layer and one or more other dielectric layers laminated together. A first signal line is provided on a first main surface, which is a main surface located on one side in a direction of lamination, of the first dielectric layer. A second signal line is provided on a second main surface, which is a main surface located on another side in the lamination direction, of the first dielectric layer so as to face the first signal line via the first dielectric layer. The second signal line is electrically connected to the first signal line. A first ground conductor is located on one side in the lamination direction than the first signal line. A second ground conductor is located on another side in the lamination direction than the second signal line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-frequency signal line, and moreparticularly to a high-frequency signal line preferably for use inhigh-frequency signal transmission.

2. Description of the Related Art

As a conventional high-frequency signal line, for example, a signal linedisclosed in WO2011/007660 is known. The signal line includes a laminatebody, a linear signal line and two ground conductors. The laminate bodyis a laminate of insulating sheets. The linear signal line is providedin the laminate body. The two ground conductors are provided in thelaminate body so as to sandwich the linear signal line in the directionof lamination. Accordingly, the linear signal line and the two groundconductors form a triplate-type stripline structure.

Each of the ground conductors has a plurality of openings at positionsover the linear signal line when viewed from the direction oflamination. As a result, little capacitance is created between thelinear signal line and each of the ground conductors. Therefore, it ispossible to reduce the distance in the direction of lamination betweenthe linear signal line and each of the ground conductors, and it ispossible to make the signal line thinner. This signal line is used, forexample, to connect two circuit boards.

The signal line disclosed in WO2011/007660 has a risk that an attempt toreduce the insertion loss results in breakage of the insulating sheetsat the time of manufacture. More specifically, in order to reduce theinsertion loss of the signal line, the thickness of the linear signalline shall be increased so that the cross-section area of the linearsignal line can be enlarged.

However, the thicker the linear signal line, the more time it takes tocomplete an etching step for processing a conductive layer into a linearsignal line. The etching step is carried out as follows: while eachinsulating sheet with a conductive layer formed entirely thereon issent, an etching solution is sprayed on the conductive layer. After theetching step, a pressure-bonding step is carried out as follows: theinsulating sheets are pressure-bonded together while the insulatingsheets are sent. Accordingly, a reduction in the processing speed at theetching step causes a reduction in the processing speed at thepressure-bonding step, and consequently, the time it takes tomanufacture the signal line is increased.

In order to improve the processing speed in the etching step, it ispossible that a more acidic etching solution is used. However, the useof a more acidic etching solution may cause damage to the insulatingsheets.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a high-frequencysignal line that prevents damage to dielectric layers while reducinginsertion loss.

A high-frequency signal line according to a preferred embodiment of thepresent invention includes a dielectric body including a firstdielectric layer and one or more other dielectric layers laminatedtogether; a first signal line provided on a first main surface of thefirst dielectric layer, the first main surface being a main surface ofthe first dielectric layer located on one side in a laminationdirection; a second signal line provided on a second main surface of thefirst dielectric layer so as to face the first signal line via the firstdielectric layer, the second main surface being a main surface of thefirst dielectric layer located on another side in the laminationdirection, and the second signal line being electrically connected tothe first signal line; a first ground conductor located on the one sidein the lamination direction than the first signal line; and a secondground conductor located on another side in the lamination directionthan the second signal line.

According to various preferred embodiments of the present invention, itis possible to prevent damage to dielectric layers while reducing theinsertion loss.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a high-frequency signal line accordingto a first preferred embodiment of the present invention.

FIG. 2 is an exploded view of a dielectric body of the high-frequencysignal line illustrated in FIG. 1.

FIG. 3 is an exploded perspective view of a line portion of thehigh-frequency signal line.

FIG. 4 is a sectional view of the high-frequency signal line along theline A-A indicated in FIG. 3.

FIG. 5 is a sectional view of the high-frequency signal line along theline B-B indicated in FIG. 3.

FIG. 6 is a perspective view of a connector of the high-frequency signalline.

FIG. 7 is a sectional view of the connector of the high-frequency signalline.

FIG. 8 is a plan view from a y-direction of an electronic deviceincluding the high-frequency signal line.

FIG. 9 is a plan view from a z-direction of the electronic deviceincluding the high-frequency signal line.

FIG. 10 is an exploded view of a dielectric body of a high-frequencysignal line according to a first modification of a preferred embodimentof the present invention.

FIG. 11 is an exploded view of a dielectric body of a high-frequencysignal line according to a second modification of a preferred embodimentof the present invention.

FIG. 12 is an exploded perspective view of a line portion of thehigh-frequency signal line.

FIG. 13 is a sectional view of the high-frequency signal line along theline A-A indicated in FIG. 12.

FIG. 14 is a sectional view of the high-frequency signal line along theline B-B indicated in FIG. 12.

FIG. 15 indicates a sectional view and a plan view of a non-curvedsignal line, and a chart indicating current distribution.

FIG. 16 indicates a sectional view and a plan view of a curved signalline and a chart indicating current distribution.

FIG. 17 is a sectional view of a high-frequency signal line according toa third modification of a preferred embodiment of the present inventioncut at an opening.

FIG. 18 is a sectional view of a dielectric body of a high-frequencysignal line according to a fourth modification of a preferred embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

High-frequency signal lines according to some preferred embodiments ofthe present invention are hereinafter described with reference to thedrawings.

First Preferred Embodiment

The structure of a high-frequency signal line 10 according to a firstpreferred embodiment of the present invention is described withreference to the drawings. FIG. 1 is a perspective view of thehigh-frequency signal line 10 according to the first preferredembodiment. FIG. 2 is an exploded view of a dielectric body 12 of thehigh-frequency signal line 10. FIG. 3 is an exploded perspective view ofa line portion 12 a of the high-frequency signal line 10. FIG. 4 is asectional view of the high-frequency signal line 10 along the line A-Aindicated in FIG. 3. FIG. 5 is a sectional view of the high-frequencysignal line 10 along the line B-B indicated in FIG. 3. In the following,the direction of lamination of the high-frequency signal line 10 isdefined as a z-direction. The lengthwise direction of the high-frequencysignal line 10 is defined as an x-direction, and the directionorthogonal to the x-direction and the z-direction is defined as ay-direction.

The high-frequency signal line 10 is preferably used, for example, in anelectronic device, such as a cellphone, to connect two high-frequencycircuits to each other. As seen in FIGS. 1 through 3, the high-frequencysignal line 10 preferably includes a dielectric body 12, externalterminals 16 a and 16 b, signal line 20 and 21, a main ground conductor22, and auxiliary ground conductor 24, via-hole conductors b1 through b4and B1 through B6, and connectors 100 a and 100 b.

The dielectric body 12 is, as illustrated in FIG. 1, a flexibleplate-shaped member extending in the x-direction when viewed from thez-direction. The dielectric body 12 includes a line portion 12 a, andconnecting portions 12 b and 12 c. As illustrated in FIG. 2, thedielectric body 12 is a laminate of a protective layer 14, dielectricsheets 18 a through 18 c and a protective layer 15 laminated in thisorder from a positive side to a negative side in the z-direction. A mainsurface of the dielectric body 12 on the positive side in thez-direction is hereinafter referred to as a top surface, and a mainsurface of the dielectric body 12 on the negative side in thez-direction is hereinafter referred to as a bottom surface.

As seen in FIG. 1, the line portion 12 a extends in the x-direction. Theconnecting portion 12 b is a rectangular or substantially rectangularportion connected to a negative end in the x-direction of the lineportion 12 a, and the connecting portion 12 c is a rectangular orsubstantially rectangular portion connected to a positive end in thex-direction of the line portion 12 a. The widths (sizes in they-direction) of the connecting portions 12 b and 12 c are greater thanthe width (size in the y-direction) of the line portion 12 a.

The dielectric sheets 18 a through 18 c, as seen in FIG. 2, extend inthe x-direction and have the same shape as the dielectric body 12 whenviewed from the z-direction. The dielectric sheets 18 a through 18 c arepreferably formed of flexible thermoplastic resin such as polyimide,liquid polymer or the like. A main surface of each of the dielectricsheets 18 a through 18 c on the positive side in the z-direction ishereinafter referred to as an upper surface, and a main surface of eachof the dielectric sheets 18 a through 18 c on the negative side in thez-direction is hereinafter referred to as a lower surface.

As illustrated in FIGS. 4 and 5, the thickness D1 of the dielectricsheet 18 a is greater than the thickness D2 of the dielectric sheet 18c. After a process of laminating the dielectric sheets 18 a through 18c, the thickness D1 preferably is, for example, within a range fromabout 50 μm to about 300 μm. In this preferred embodiment, the thicknessD1 preferably is about 150 μm, for example. The thickness D2 preferablyis, for example, within a range from about 10 μm to about 100 μm. Inthis preferred embodiment, the thickness D2 preferably is about 50 μm,for example. The thickness D3 of the dielectric sheet 18 b preferablyis, for example, within a range from about 3 μm to about 50 μm. In thispreferred embodiment, the thickness D3 preferably is about 12.5 μm, forexample.

The dielectric sheet 18 a, as illustrated in FIG. 2, includes a lineportion 18 a-a, and connecting portions 18 a-b and 18 a-c. Thedielectric sheet 18 b, as illustrated in FIG. 2, includes a line portion18 b-a, and connecting portions 18 b-b and 18 b-c. The dielectric sheet18 c includes a line portion 18 c-a, and connecting portions 18 c-b and18 c-c. The line portions 18 a-a, 18 b-a and 18 c-a constitute the lineportion 12 a. The connecting portions 18 a-b, 18 b-b and 18 c-bconstitute the connecting portion 12 b. The connecting portions 18 a-c,18 b-c and 18 c-c constitute the connecting portion 12 c.

The signal line 20 is, as seen in FIGS. 2 and 3, a conductor provided inthe dielectric body 12, and the signal line 20 is configured to transmita high-frequency signal. In this preferred embodiment, the signal line20 is a linear conductor provided on the upper surface of the dielectricsheet 18 b and extends straight in the x-direction. The negative end inthe x-direction of the signal line 20 is, as seen in FIG. 2, located inthe center of the connecting portion 18 b-b. The positive end in thex-direction of the signal line 20 is, as seen in FIG. 2, located in thecenter of the connecting portion 18 b-c. The signal line 20 preferablyis preferably formed of a metal material with a low specific resistanceincluding silver or copper. The statement that the signal line 20 isprovided on the upper surface of the dielectric sheet 18 b means thatthe signal line 20 is preferably formed by plating the upper surface ofthe dielectric sheet 18 b with a metal foil and by patterning the metalfoil or that the signal line 20 is preferably formed by applying a metalfoil on the upper surface of the dielectric sheet 18 b and by patterningthe metal foil. The surface of the signal line 20 is smoothened, andtherefore, the surface of the signal line 20 in contact with thedielectric sheet 18 b has a greater surface roughness than the surfaceof the signal line 20 out of contact with the dielectric sheet 18 b. Thesurface roughness means the calculated average roughness Ra provided inJIS B 0601-2001 (compliant with ISO4287-1997), and the same shall applyhereinafter.

The signal line 21 is, as seen in FIGS. 2 and 3, a conductor provided inthe dielectric body 12, and the signal line 21 is configured to transmita high-frequency signal. In this preferred embodiment, the signal line21 is a linear conductor provided on the lower surface of the dielectricsheet 18 b and extends straight in the x-direction. The negative end inthe x-direction of the signal line 21 is, as seen in FIG. 2, located inthe center of the connecting portion 18 b-b. The positive end in thex-direction of the signal line 21 is, as seen in FIG. 2, located in thecenter of the connecting portion 18 b-c. The signal line 21 ispreferably formed of a metal material with a low specific resistanceincluding silver or copper. The statement that the signal line 21 isprovided on the lower surface of the dielectric sheet 18 b means thatthe signal line 21 is preferably formed by plating the lower surface ofthe dielectric sheet 18 b with a metal foil and by patterning the metalfoil or that the signal line 21 is preferably formed by applying a metalfoil on the lower surface of the dielectric sheet 18 b and by patterningthe metal foil. The surface of the signal line 21 is smoothened, andtherefore, the surface of the signal line 21 in contact with thedielectric sheet 18 b (upper surface of the signal line 21) has agreater surface roughness than the surface of the signal line 21 out ofcontact with the dielectric sheet 18 b (lower surface of the signal line21).

As seen in FIG. 5, the signal line 20 and the signal line 21 face eachother via the dielectric sheet 18 b. In this preferred embodiment, thesignal lines 20 and 21 have the same shape and are located on the sameposition when viewed from the z-direction. The signal lines 20 and 21 donot need to be completely identical to each other and do not need to belocated on quite the same position when viewed from the z-direction.However, in order to permit the signal lines 20 and 21 to define andfunction substantially as one signal line, it is preferred that thesignal lines 20 and 21 have substantially the same shape andsubstantially overlap each other.

The main ground conductor 22 is, as seen in FIG. 2, a continuousconductor layer located farther in the positive z-direction than thesignal line 20. More specifically, the main ground conductor 22 isarranged on the upper surface of the dielectric sheet 18 a so as to facethe signal line 20 via the dielectric sheet 18 a. The main groundconductor 22 does not have openings at positions over the signal line20. The main ground conductor 22 is preferably formed of a metalmaterial with a low specific resistance including silver or copper. Thestatement that the main ground conductor 22 is provided on the uppersurface of the dielectric sheet 18 a means that the main groundconductor is preferably formed by plating the upper surface of thedielectric sheet 18 a with a metal foil and by patterning the metal foilor that the main ground conductor 22 is preferably formed by applying ametal foil on the upper surface of the dielectric sheet 18 a and bypatterning the metal foil. The surface of the main ground conductor 22is smoothened, and therefore, the surface of the main ground conductor22 in contact with the dielectric layer 18 a has a greater surfaceroughness than the surface of the main ground conductor 22 out ofcontact with the dielectric layer 18 a.

The main ground conductor 22, as illustrated in FIG. 2, includes a lineportion 22 a, and terminal portions 22 b and 22 c. The line portion 22 ais provided on the upper surface of the line portion 18 a-a to extend inthe x-direction. The terminal portion 22 b is provided on the uppersurface of the connecting portion 18 a-b and is rectangular orsubstantially rectangular ring-shaped. The terminal portion 22 b isconnected to the negative end in the x-direction of the line portion 22a. The terminal portion 22 c is provided on the upper surface of theconnecting portion 18 a-c and is rectangular or substantiallyrectangular ring-shaped. The terminal portion 22 c is connected to thepositive end in the x-direction of the line portion 22 a.

The auxiliary ground conductor 24 is, as seen in FIG. 2, a conductorlayer located farther in the negative z-direction than the signal line21. More specifically, the auxiliary ground conductor 24 is provided onthe lower surface of the dielectric sheet 18 c so as to face the signalline 21 via the dielectric sheet 18 c. The auxiliary ground conductor 24is preferably formed of a metal material with a low specific resistanceincluding silver or copper. The statement that the auxiliary groundconductor 24 is provided on the lower surface of the dielectric sheet 18c means that the auxiliary ground conductor 24 is preferably formed byplating the lower surface of the dielectric sheet 18 c with a metal foiland by patterning the metal foil or that the auxiliary ground conductor24 is preferably formed by applying a metal foil on the lower surface ofthe dielectric sheet 18 c and by patterning the metal foil. The surfaceof the auxiliary ground conductor 24 is smoothened. Therefore, asindicated in FIGS. 4 and 5, the surface of the auxiliary groundconductor 24 in contact with the dielectric layer 18 c has a greatersurface roughness than the surface of the auxiliary ground conductor 24out of contact with the dielectric layer 18 c.

The auxiliary ground conductor 24, as illustrated in FIG. 2, includes aline portion 24 a, and terminal portions 24 b and 24 c. The line portion24 a is provided on the lower surface of the line portion 18 c-a toextend in the x-direction. The terminal portion 24 b is provided on thelower surface of the connecting portion 18 c-b and is rectangular orsubstantially rectangular ring-shaped. The terminal portion 24 b isconnected to the negative end in the x-direction of the line portion 24a. The terminal portion 24 c is provided on the lower surface of theconnecting portion 18 c-c and is rectangular or substantiallyrectangular ring-shaped. The terminal portion 24 c is connected to thepositive end in the x-direction of the line portion 24 a.

As seen in FIGS. 2 and 3, the line portion 24 a includes rectangular orsubstantially rectangular openings 30 aligned in the x-direction.Accordingly, the line portion 24 a preferably has a ladder-shapedconfiguration. In the auxiliary ground conductor 24, intervals betweenthe openings 30 are referred to as bridges 60. Each of the bridges 60extends in the y-direction. When viewed from the z-direction, theopenings 30 and the bridges 60 are alternately arranged to be overlappedwith the signal line 20. In this preferred embodiment, the signal line20 extends in the x-direction while crossing the centers of the openings30 and the bridges 60.

As seen in FIG. 2, the external terminal 16 a is a rectangular orsubstantially rectangular conductor provided in the center of the uppersurface of the connecting portion 18 a-b of the dielectric sheet 18 a.Therefore, when viewed from the z-direction, the external terminal 16 ais over the respective negative ends in the x-direction of the signallines 20 and 21. As seen in FIG. 2, the external terminal 16 b is arectangular or substantially rectangular conductor provided in thecenter of the upper surface of the connecting portion 18 a-c of thedielectric sheet 18 a. Therefore, when viewed from the z-direction, theexternal terminal 16 b is over the respective positive ends in thex-direction of the signal lines 20 and 21. The external terminals 16 aand 16 b are preferably formed of a metal material with a low specificresistance including silver or copper. The external terminals 16 a and16 b preferably are plated with Ni/Au. The statement that the externalterminals 16 a and 16 b are provided on the upper surface of thedielectric sheet 18 a means that the external terminals 16 a and 16 bare preferably formed by plating the upper surface of the dielectricsheet 18 a with a metal foil and by patterning the metal foil or thatthe external terminals 16 a and 16 b are preferably formed by applying ametal foil on the upper surface of the dielectric sheet 18 a and bypatterning the metal foil. The surfaces of the external terminals 16 aand 16 b are smoothened, and therefore, the respective surfaces of theexternal terminals 16 a and 16 b in contact with the dielectric layer 18a have a greater surface roughness than the respective surfaces of theexternal terminals 16 a and 16 b out of contact with the dielectriclayer 18 a.

The external terminals 16 a and 16 b, the signal lines 20 and 21, themain ground conductor 22 and the auxiliary ground conductor 24preferably have a same or substantially same thickness. The thickness ofthe external terminals 16 a and 16 b, the signal lines 20 and 21, themain ground conductor 22 and the auxiliary ground conductor 24preferably are, for example, within a range from about 10 μm to about 20μm.

As described above, the signal lines 20 and 21 are sandwiched betweenthe main ground conductor 22 and the auxiliary ground conductor 24 fromboth sides in the z-direction. Thus, the signal lines 20 and 21, themain ground conductor 22 and the auxiliary ground conductor 24 define atriplate stripline structure. As illustrated in FIGS. 4 and 5, theinterval (distance in the z-direction) between the signal line 20 andthe main ground conductor 22 is equal or substantially equal to thethickness D1 of the dielectric sheet 18 a, and preferably is, forexample, within a range from about 50 μm to about 300 μm. In thispreferred embodiment, the interval between the signal line 20 and themain ground conductor 22 preferably is about 150 μm, for example. Theinterval (distance in the z-direction) between the signal line 21 andthe auxiliary ground conductor 24 is equal or substantially equal to thethickness D2 of the dielectric sheet 18 c, and preferably is, forexample, within a range from about 10 μm to about 100 μm. In thispreferred embodiment, the interval between the signal line 21 and theauxiliary ground conductor 24 preferably is about 50 μm, for example.Thus, the distance in the z-direction between the signal line 20 and themain ground conductor 22 is greater than the distance in the z-directionbetween the signal line 21 and the auxiliary ground conductor 24.

The via-hole conductors B1 are, as seen in FIG. 2, pierced in thedielectric sheet 18 a in the z-direction. The via-hole conductors B1 arelocated farther in the positive y-direction than the signal lines 20 and21, and are aligned in the x-direction at uniform intervals. Thevia-hole conductors B2 are, as seen in FIG. 2, pierced in the dielectricsheet 18 b in the z-direction. The via-hole conductors B2 are locatedfarther in the positive y-direction than the signal lines 20 and 21, andare aligned in the x-direction at uniform intervals. The via-holeconductors B3 are, as seen in FIG. 2, pierced in the dielectric sheet 18c in the z-direction. The via-hole conductors B3 are located farther inthe positive y-direction than the signal lines 20 and 21, and arealigned in the x-direction at uniform intervals. The via-hole conductorsB1 are connected to the respectively adjacent via-hole conductors B2,and the via-hole conductors B2 are connected to the respectivelyadjacent via-hole conductors B3. Accordingly, each connected set ofvia-hole conductors B1 through B3 defines and serves as one via-holeconductor. The respective positive ends in the z-direction of thevia-hole conductors B1 are connected to the main ground conductor 22.The respective negative ends in the z-direction of the via-holeconductors B3 are connected to the auxiliary ground conductor 24, andmore specifically to the respective positive sides in the y-direction ofthe bridges 60. The via-hole conductors B1 through B3 are preferablyformed by filling via holes made in the dielectric sheets 18 a through18 c with conductive paste including silver, tin, copper or the like andby solidifying the conductive paste.

The via-hole conductors B4 are, as seen in FIG. 2, pierced in thedielectric sheet 18 a in the z-direction. The via-hole conductors B4 arelocated farther in the negative y-direction than the signal lines 20 and21, and are aligned in the x-direction at uniform intervals. Thevia-hole conductors B5 are, as seen in FIG. 2, pierced in the dielectricsheet 18 b in the z-direction. The via-hole conductors B5 are locatedfarther in the negative y-direction than the signal lines 20 and 21, andare aligned in the x-direction at uniform intervals. The via-holeconductors B6 are, as seen in FIG. 2, pierced in the dielectric sheet 18c in the z-direction. The via-hole conductors B6 are located farther inthe negative y-direction than the signal lines 20 and 21, and arealigned in the x-direction at uniform intervals. The via-hole conductorsB4 are connected to the respectively adjacent via-hole conductors B5,and the via-hole conductors B5 are connected to the respectivelyadjacent via-hole conductors B6. Accordingly, each connected set ofvia-hole conductors B4 through B6 defines and serves as one via-holeconductor. The respective positive ends in the z-direction of thevia-hole conductors B4 are connected to the main ground conductor 22.The respective negative ends in the z-direction of the via-holeconductors B6 are connected to the auxiliary ground conductor 24, andmore specifically to the respective negative sides in the y-direction ofthe bridges 60. The via-hole conductors B4 through B6 are preferablyformed by filling via holes made in the dielectric sheets 18 a through18 c with conductive paste including silver, tin, copper or the like andby solidifying the conductive paste.

The via-hole conductor b1, as seen in FIG. 2, is pierced in thedielectric sheet 18 a in the z-direction so as to connect the externalterminal 16 a to the negative end in the x-direction of the signal line20. The via-hole conductor b3, as seen in FIG. 2, is pierced in thedielectric sheet 18 b in the z-direction so as to connect the negativeend in the x-direction of the signal line 20 to the negative end in thex-direction of the signal line 21. The via-hole conductor b2, as seen inFIG. 2, is pierced in the dielectric sheet 18 a in the z-direction so asto connect the external terminal 16 b to the positive end in thex-direction of the signal line 20. The via-hole conductor b4, as seen inFIG. 2, is pierced in the dielectric sheet 18 b in the z-direction so asto connect the positive end in the x-direction of the signal line 20 tothe positive end in the x-direction of the signal line 21. Accordingly,the signal lines 20 and 21 are connected between the external terminal16 a and 16 b, and the signal line 20 and the signal line 21 areelectrically connected to each other. The via-hole conductors b1 throughb4 are preferably formed by filling via holes made in the dielectricsheets 18 a and 18 b with conductive paste including silver, tin, copperor the like and by solidifying the conductive paste.

The protective layer 14 is an insulating layer covering substantiallythe entire upper surface of the dielectric sheet 18 a. Accordingly, theprotective layer 14 covers the main ground conductor 22. The protectivelayer 14 is preferably formed of, for example, flexible resin such as aresist material.

The protective layer 14, as illustrated in FIG. 2, includes a lineportion 14 a, and connecting portions 14 b and 14 c. The line portion 14a covers substantially the entire upper surface of the line portion 18a-a and accordingly covers the line portion 22 a of the main groundconductor 22.

The connecting portion 14 b is connected to the negative end in thex-direction of the line portion 14 a and covers the upper surface of theconnecting portion 18 a-b. However, the connecting portion 14 b includesopenings Ha through Hd. The opening Ha is a rectangular or substantiallyrectangular opening made in the center of the connecting portion 14 b.The external terminal 16 a is exposed to outside through the opening Ha.The opening Hb is a rectangular or substantially rectangular openinglocated farther in the positive y-direction than the opening Ha. Theopening Hc is a rectangular or substantially rectangular opening locatedfarther in the negative x-direction than the opening Ha. The opening Hdis a rectangular or substantially rectangular opening located farther inthe negative y-direction than the opening Ha. The terminal portion 22 bis exposed to outside through the openings Hb through Hd and defines andfunctions as an external terminal.

The connecting portion 14 c is connected to the positive end in thex-direction of the line portion 14 a and covers the upper surface of theconnecting portion 18 a-c. However, the connecting portion 14 c includesopenings He through Hh. The opening He is a rectangular or substantiallyrectangular opening made in the center of the connecting portion 14 c.The external terminal 16 b is exposed to outside through the opening He.The opening Hf is a rectangular or substantially rectangular openinglocated farther in the positive y-direction than the opening He. Theopening Hg is a rectangular or substantially rectangular opening locatedfarther in the positive x-direction than the opening He. The opening Hhis a rectangular or substantially rectangular opening located farther inthe negative y-direction than the opening He. The terminal portion 22 cis exposed to outside through the openings Hf through Hh and defines andfunctions as an external terminal.

The protective layer 15 is an insulating layer covering substantiallythe entire lower surface of the dielectric sheet 18 c. Accordingly, theprotective layer 15 covers the auxiliary ground conductor 24. Theprotective layer 15 is preferably formed of, for example, flexible resinsuch as a resist material.

In the high-frequency signal line 10 having the structure describedabove, the characteristic impedance of the signal lines 20 and 21changes cyclically between an impedance value Z1 and an impedance valueZ2. More specifically, in areas A1 where the signal lines 20 and 21 areover the openings 30, relatively small capacitance is created betweenthe signal lines 20 and 21, and the auxiliary ground conductor 24.Accordingly, the characteristic impedance of the signal lines 20 and 21in the areas A1 is a relatively high value Z1.

In areas A2 where the signal lines 20 extend over the bridges 60, on theother hand, a relatively large capacitance is created between the signallines 20 and 21, and the auxiliary ground conductor 24. Accordingly, thecharacteristic impedance of the signal lines 20 and 21 in the areas A2is a relatively low value Z2. In this regard, the areas A1 and the areasA2 are arranged alternately in the x-direction, and therefore, thecharacteristic impedance of the signal lines 20 and 21 changescyclically between the value Z1 and the value Z2. The impedance value Z1is, for example, about 55Ω, and the impedance value Z2 is, for example,about 45Ω. The average characteristic impedance of the signal lines 20and 21 as a whole is, for example, about 50Ω.

The connectors 100 a and 100 b are, as illustrated in FIG. 1, mounted onthe top surfaces of the connecting portions 12 b and 12 c, respectively.The connectors 100 a and 100 b preferably have the same structure, andin the following, the structure of the connector 100 b is described asan example. FIG. 6 is a perspective view of the connector 100 b of thehigh-frequency signal line 10. FIG. 7 is a sectional view of theconnector 100 b of the high-frequency signal line 10.

The connector 100 b, as illustrated in FIGS. 1, 6 and 7, preferablyincludes a connector body 102, external terminals 104 and 106, a centralconductor 108 and an external conductor 110. The connector body 102preferably is in the shape of a rectangular or substantially rectangularplate with a cylinder connected thereon, and is preferably formed of aninsulating material such as resin.

The external terminal 104 is provided on the surface of the plate-shapedportion of the connector body 102 on the negative side in thez-direction so as to face the external terminal 16 b. The externalterminal 106 is provided on the surface of the plate-shaped portion ofthe connector body 102 on the negative side in the z-direction so as toface the terminal conductor 22 c exposed through the openings Hf throughHh.

The central conductor 108 is located in the center of the cylindricalportion of the connector body 102 and is connected to the externalterminal 104. The central conductor 108 is a signal terminal at which ahigh-frequency signal is input or output. The external conductor 110 isprovided on the inner surface of the cylindrical portion of theconnector body 102 and is connected to the external terminal 106. Theexternal conductor 110 is aground terminal that is maintained at aground potential.

The connector 100 b having the structure described above is, asillustrated in FIGS. 6 and 7, mounted on the top surface of theconnecting portion 12 c such that the external terminal 104 is connectedto the external terminal 16 b and such that the external terminal 106 isconnected to the terminal conductor 22 c. As a result, the signal lines20 and 21 are electrically connected to the central conductor 108, andthe main ground conductor 22 and the auxiliary ground conductor 24 areelectrically connected to the external conductor 110.

The high-frequency signal line 10 preferably is used in the followingway. FIG. 8 is a plan view from the y-direction of an electronic device200 including the high-frequency signal line 10. FIG. 9 is a plan viewfrom the z-direction of the electronic device 200 including thehigh-frequency signal line 10.

The electronic device 200 preferably includes the high-frequency signalline 10, circuit boards 202 a and 202 b, receptacles 204 a and 204 b, abattery pack (metal object) 206, and a case 210.

In the circuit board 202 a, for example, a transmitting circuit or areceiving circuit including an antenna is provided. In the circuit board202 b, for example, a feed circuit is provided. The battery pack 206 is,for example, a lithium-ion secondary battery, and the surface of thebattery pack 206 is covered by a metal cover. The circuit board 202 a,the battery pack 206 and the circuit board 202 b are arranged in thisorder from the negative side to the positive side in the x-direction.

The receptacles 204 a and 204 b are provided on respective main surfacesof the circuit boards 202 a and 202 b on the negative side in thez-direction. The connectors 100 a and 100 b are connected to thereceptacles 204 a and 204 b respectively. As a result, a high-frequencysignal with a frequency of, for example, 2 GHz to be transmitted betweenthe circuit boards 202 a and 202 b is applied to the central conductors108 of the connectors 100 a and 100 b through the receptacles 204 a and204 b. The respective external conductors 110 of the connectors 100 aand 100 b are maintained at the ground potential through the circuitboards 202 a and 202 b, and the receptacles 204 a and 204 b. In thisway, the high-frequency signal line 10 connects the circuit boards 202 aand 202 b to each other.

In this state, the top surface of the dielectric body 12 (morespecifically, the protective layer 14) is in contact with the battery206, and the dielectric body 12 is fixed to the battery pack 206preferably by an adhesive.

With reference to the drawings, a manufacturing method of thehigh-frequency signal line 10 is described below. In the following, amanufacturing method of one high-frequency signal line 10 is describedas an example. Practically, however, by laminating large-size dielectricsheets and by cutting the laminate, a plurality of high-frequency signallines are produced at one time.

First, dielectric sheets, each preferably formed of thermoplastic resinand having a copper foil (metal film) entirely on one main surface, areprepared as the dielectric sheets 18 a and 18 c. Specifically, copperfoils are applied to respective one main surface of the dielectricsheets 18 a and 18 c. The surfaces of the copper foils are, for example,galvanized for corrosion proof and thus are smoothened. Each of thedielectric sheets 18 a and 18 c is lined with copper and obtains anon-fixation surface (shiny surface) with a small surface roughness anda fixation surface (mat surface) with a great surface roughness. Thedielectric sheets 18 a through 18 c are preferably formed of liquidpolymer. The thicknesses of the copper foils are preferably within arange from about 10 μm to about 20 μm, for example.

Also, a dielectric sheet preferably formed of thermoplastic resin andhaving copper foils (metal films) provided entirely on both mainsurfaces is prepared as the dielectric sheet 18 b. Specifically, copperfoils are applied to the both main surfaces of the dielectric sheet 18b. The surfaces of the copper foils are, for example, galvanized forcorrosion proof and thus are smoothened. The dielectric sheet 18 b ispreferably formed of liquid polymer. The thicknesses of the copper foilsare preferably within a range from about 10 μm to about 20 μm, forexample.

Next, the external terminals 16 a and 16 b, and the main groundconductor 22 as illustrated in FIG. 2 are preferably formed on the uppersurface of the dielectric sheet 18 a preferably by patterning the copperfoil formed on the upper surface of the dielectric sheet 18 a.Specifically, resists having identical shapes to the external terminals16 a and 16 b, and the main ground conductor 22 are printed on thecopper foil on the upper surface of the dielectric sheet 18 a. Then, thecopper foil is etched, so that the portions of the copper foil notcovered by the resists are removed. Thereafter, a resist remover issprayed so as to remove the resists. In this way, the external terminals16 a and 16 b, and the main ground conductor 22 as illustrated in FIG. 2are preferably formed on the upper surface of the dielectric sheet 18 aby photolithography, for example.

Next, the signal line 20 as illustrated in FIG. 2 is formed on the uppersurface of the dielectric sheet 18 b, and the signal line 21 is formedon the lower surface of the dielectric sheet 18 b. Further, theauxiliary ground conductor 24 as illustrated in FIG. 2 is formed on thelower surface of the dielectric sheet 18 c. The process for forming thesignal line 20, the process for forming the signal line 21 and theprocess for forming the auxiliary ground conductor 24 are the same asthe process for forming the external terminals 16 a and 16 b, and themain ground conductor 22, and descriptions of the processes are omittedhere.

Next, the dielectric sheets 18 a through 18 c are exposed to laser beamssuch that through holes are made in the dielectric sheets 18 a through18 c at the positions of the via-hole conductors b1 through b4 and B1through B6. Thereafter, conductive paste is filled in the through holes,and thus, the via-hole conductors b1 through b4 and B1 through B6 areformed.

Next, the dielectric sheets 18 a through 18 c are laminated in thisorder from the positive side to the negative side in the z-direction soas to form the dielectric body 12. In this regard, heat and pressure areapplied to the dielectric sheets 18 a through 18 c from the both sidesin the z-direction, and thus, the dielectric sheets 18 a through 18 care pressure-bonded together.

Next, resin (resist) paste is applied to the upper surface of thedielectric sheet 18 a as illustrated in FIG. 2 by screen printing. Thus,the protective layer 14 covering the upper surface of the main groundconductor 22 is formed on the upper surface of the dielectric sheet 18a.

Next, resin (resist) paste is applied to the lower surface of thedielectric sheet 18 c as illustrated in FIG. 2 by screen printing. Thus,the protective layer 15 covering the lower surface of the auxiliaryground conductor 24 is formed on the lower surface of the dielectricsheet 18 c.

Lastly, the connector 100 a is mounted on the connecting portion 12 band soldered to the external terminal 16 a and the terminal conductor 22b, and the connector 100 b is mounted on the connecting portion 12 c andsoldered to the external terminal 16 b and the terminal conductor 22 c.Through the process above, the high-frequency signal line 10 asillustrated in FIG. 1 is obtained.

In the high-frequency signal line 10 having the structure describedabove, the insertion loss is significantly reduced. More specifically,in the high-frequency signal line 10, the signal line 20 is provided onthe upper surface of the dielectric sheet 18 b, and the signal line 21is provided on the lower surface of the dielectric sheet 18 b. Thesignal lines 20 and 21 face each other and are electrically connected toeach other. Accordingly, the signal lines 20 and 21 constitute onesignal transmission route. The thickness of the signal transmissionroute including the signal lines 20 and 21 is the total of the thicknessof the signal line and the thickness of the signal line 21. Thus, thecross-sectional area of the signal transmission route is increasedbecause the signal transmission route includes the signal lines 20 and21. Consequently, the insertion loss of the high-frequency signal line10 is significantly reduced.

In the high-frequency signal line 10, there is another reason as followsfor the reduction in the insertion loss. Specifically, when ahigh-frequency signal flows in the signal line 20, a current flowsintensively in the superficial portion of the signal line 20 by the skineffect. With respect to the signal line 20, the current flowsintensively especially on and near the surface facing the main groundconductor 22 (that is, the surface out of contact with the dielectricsheet 18 b). More specifically, in the high-frequency signal line 10,the surface roughness of the surface of the signal line 20 out ofcontact with the dielectric sheet 18 b is smaller than that of thesurface of the signal line in contact with the dielectric sheet 18 b.Therefore, the proportion of conductive material in an area from thesurface of the signal line 20 out of contact with the dielectric sheet18 b to a position at a specified depth (an area where the current flowsintensively) is higher than the proportion of conductive material in anarea from the surface of the signal line 20 in contact with thedielectric sheet 18 b to the position at the specified depth.Accordingly, the portion of the signal line 20 near the surface out ofcontact with the dielectric sheet 18 b is easier to pass a current thanthe portion of the signal line 20 near the surface in contact with thedielectric sheet 18 b. Consequently, in the high-frequency signal line10, the insertion loss is significantly reduced. Further, the samephenomenon occurs on the signal line 21.

In the high-frequency signal line 10, damage to the dielectric sheets 18a and 18 b is prevented. Specifically, the signal transmission routeincludes the signal lines 20 and 21. The signal line 20 is preferablyformed by etching a conductive layer having a thickness equal orsubstantially equal to the thickness of the signal line 20. In the sameway, the signal line 21 is preferably formed by etching a conductivelayer having a thickness equal or substantially equal to the thicknessof the signal line 21. Accordingly, in order to form a signaltransmission route of the signal lines 20 and 21, it is not necessary toetch a conductive layer having a thickness equal or substantially equalto the total of the thickness of the signal line 20 and the thickness ofthe signal line 21, and it is not necessary to use a more acid etchingsolution. Thus, the risk that the dielectric sheets 18 a and 18 c aredamaged during a process of forming a signal transmission route of thesignal lines 20 and 21 is diminished.

Also, the high-frequency signal line 10 is easy to bend. Specifically,when the high-frequency signal line 10 is bent, for example, the signalline 20 located in the outer periphery stretches, and the signal line 21located in the inner periphery compresses. Accordingly, the signal lines20 and 21 come out of alignment with each other. More specifically, theflexible dielectric sheet 18 b is provided between the signal lines 20and 21. Thus, when the signal line 20 stretches and the signal linecompresses, the flexible dielectric sheet 18 b deforms. Accordingly,when the high-frequency signal line 10 is bent, the signal lines 20 and21 readily come out of alignment with each other. Thus, thehigh-frequency signal line 10 is easy to bend.

Further, it is possible to make the high-frequency signal line 10thinner. More specifically, when the high-frequency signal line 10 isviewed from the z-direction, in the areas A1, the signal lines 20 and 21are not over the auxiliary ground conductor 24. Accordingly, littlecapacitance is created between the signal lines 20 and 21, and theauxiliary ground conductor 24. Therefore, even a reduction in thedistance between the signal lines 20 and 21, and the auxiliary groundconductor 24 will not cause a significant increase in the capacitancebetween the signal lines 20 and 21, and the auxiliary ground conductor24 and will not result in a significant shift of the characteristicimpedance of the signal lines 20 and 21 from a designed value (forexample, about 50Ω). Thus, it is possible to make the high-frequencysignal line 10 thinner while maintaining the characteristic impedance ofthe signal lines 20 and 21 at a designed value.

Even when the high-frequency signal line 10 is fixed to a metal objectsuch as the battery pack 206, a change in the characteristic impedanceof the signal lines 20 and 21 is prevented. More specifically, thehigh-frequency signal line 10 is fixed to the battery pack 206 such thatthe continuous main ground conductor 22 is located between the signallines 20 and 21, and the battery pack 206. Therefore, there is no riskthat the signal lines 20 and 21 face the battery pack 206 via openings,and capacitance is prevented from occurring between the signal lines 20and 21, and the battery pack 206. Accordingly, it is unlikely that thecharacteristic impedance of the signal lines 20 and 21 is reduced due tothe fixation of the high-frequency signal line 10 to the battery pack206.

First Modification

The structure of a high-frequency signal line 10 a according to a firstmodification of a preferred embodiment of the present invention isdescribed with reference to the drawings. FIG. 10 is an exploded view ofthe dielectric body 12 of the high-frequency signal line 10 a accordingto the first modification. The appearance of the high-frequency signalline 10 a is as illustrated in FIG. 1.

The high-frequency signal line 10 a is different from the high-frequencysignal line 10 in that the main ground conductor 22 includes openings34.

As seen in FIG. 10, the line portion 22 a of the main ground conductor22 includes rectangular or substantially rectangular openings 34 alignedin the x-direction. Accordingly, the line portion 24 a is shaped like aladder. In the main ground conductor 22, intervals between the openings34 are referred to as bridges 62. Each of the bridges 62 extends in they-direction. When viewed from the z-direction, the openings 34 and thebridges 62 are alternately arranged to be overlapped with the signalline 20. In this preferred embodiment, the signal line 20 extends in thex-direction while crossing the centers of the openings 34 and thebridges 62.

The openings 34 are smaller than the openings 30. Specifically, thelength (size in the x-direction) of each of the openings 34 is smallerthan the length (size in the x-direction) of each of the openings 30.The width (size in the y-direction) of each of the openings 34 issmaller than the width (size in the y-direction) of the each of theopenings 30. When viewed from the z-direction, the outer edges of theopenings 30 do not overlap the outer edges of the openings 34. Whenviewed from the z-direction, the openings 34 are inside the respectiveouter edges of the openings 30.

In the high-frequency signal line 10 a having the structure describedabove, the insertion loss is significantly reduced for the same reasondescribed above in connection with the high-frequency signal line 10.

In the high-frequency signal line 10 a, damage to the dielectric sheets18 a and 18 b is prevented for the same reason described above inconnection with the high-frequency signal line 10.

The high-frequency signal line 10 a is easy to bend for the same reasondescribed above in connection with the high-frequency signal line 10.

It is possible to make the high-frequency signal line 10 a thinner forthe same reason described above in connection with the high-frequencysignal line 10.

Moreover, it is possible to further reduce the insertion loss of thehigh-frequency signal line 10 a for the following reason. In thehigh-frequency signal line 10 a, when a current i1 flows in the signallines 20 and 21, a countercurrent i2 flows in the main ground conductor22, and a countercurrent i3 flows in the auxiliary ground conductor 24.The countercurrents i2 and i3 flow along the outer edges of the openings30 and 34 by skin effect. In the high-frequency signal line 10 a, theouter edges of the openings 30 do not overlap the outer edges of theopenings 34. Accordingly, the flow path of the countercurrent i2 isspaced from the flow path of the countercurrent i3, and coupling betweenthe countercurrent i2 and the countercurrent i3 is weak. Therefore, thecurrent i1 flows easily. Thus, the insertion loss of the high-frequencysignal line 10 a is further reduced.

Second Modification

The structure of a high-frequency signal line 10 b according to a secondmodification of a preferred embodiment of the present invention isdescribed with reference to the drawings. FIG. 11 is an exploded view ofthe dielectric body 12 of the high-frequency signal line 10 b accordingto the second modification. FIG. 12 is an exploded perspective view ofthe line portion 12 a of the high-frequency signal line 10 b. FIG. 13 isa sectional view of the high-frequency signal line 10 b along the lineA-A indicated in FIG. 12. FIG. 14 is a sectional view of thehigh-frequency signal line 10 b along the line B-B indicated in FIG. 12.The appearance of the high-frequency signal line 10 b is as illustratedin FIG. 1.

The high-frequency signal line 10 b is different from the high-frequencysignal line 10 in that the line width of the signal line 20 is differentfrom the line width of the signal line 21 and that through-holeconductors T1 through T4 are used. Specifically, as seen in FIGS. 11 and12, the line width of the signal line 20 is greater than the line widthof the signal line 21. When viewed from the z-direction, the signal line21 is located inside the signal line 20. The signal line 21 does notneed to be located completely inside the signal line 20 when viewed fromthe z-direction. However, in order to permit the signal lines 20 and 21to function substantially as one signal line, it is preferred that thesignal line 21 is located substantially inside the signal line 20 whenviewed from the z-direction.

During a laminate/pressure-bonding process for forming the dielectricbody 12 of the high-frequency signal line 10 b, the signal line 20deforms. More specifically, as seen in FIGS. 13 and 14, the signal line20 curves so as to protrude in the positive z-direction.

The through-hole conductors T1 are, as seen in FIG. 11, pierced in thedielectric sheets 18 a through 18 c in the z-direction. The through-holeconductors T1 are located farther in the positive y-direction than thesignal lines 20 and 21, and are aligned in the x-direction at uniformintervals. The respective positive ends in the z-direction of thethrough-hole conductors T1 are connected to the main ground conductor22. The respective negative ends in the z-direction of the through-holeconductors T1 are connected to the auxiliary ground conductor 24.Accordingly, the through-hole conductors T1 connect the main groundconductor 22 and the auxiliary ground conductor 24 to each other. Thethrough-hole conductors T1 are preferably formed by piercing throughholes in the dielectric sheets 18 a through 18 c and by forming metalfilms including nickel, gold or the like on inner peripheral surfaces ofthe through holes by plating.

The through-hole conductors T2 are, as seen in FIG. 11, pierced in thedielectric sheets 18 a through 18 c in the z-direction. The through-holeconductors T2 are located farther in the negative y-direction than thesignal lines 20 and 21, and are aligned in the x-direction at uniform orsubstantially uniform intervals. The respective positive ends in thez-direction of the through-hole conductors T2 are connected to the mainground conductor 22. The respective negative ends in the z-direction ofthe through-hole conductors T2 are connected to the auxiliary groundconductor 24. Accordingly, the through-hole conductors T2 connect themain ground conductor 22 and the auxiliary ground conductor 24 to eachother. The through-hole conductors T2 are preferably formed by piercingthrough holes in the dielectric sheets 18 a through 18 c and by formingmetal films including nickel, gold or the like on inner peripheralsurfaces of the through holes by plating.

The through-hole conductor T3, as seen in FIG. 11, is pierced in thedielectric sheets 18 a through 18 c in the z-direction so as to connectthe external terminal 16 a to the negative end in the x-direction of thesignal line 20 and farther to the negative end in the y-direction of thesignal line 21. The through-hole conductor T4, as seen in FIG. 11, ispierced in the dielectric sheets 18 a through 18 c in the z-direction soas to connect the external terminal 16 b to the positive end in thex-direction of the signal line 20 a and farther to the positive end inthe x-direction of the signal line 21. Accordingly, the signal lines 20and 21 are connected between the external terminal 16 a and 16 b, andthe signal line 20 and the signal line 21 are electrically connected toeach other. The through-hole conductors T3 and T4 are preferably formedby piercing through holes in the dielectric sheets 18 a through 18 c andby forming metal films including nickel, gold or the like on innerperipheral surfaces of the through holes by plating.

The protective layer 14 has openings O1 and O2 at positions over thethrough-hole conductors T1 and T2. The protective layer 15 includesopenings O3 through O6 at positions underneath the through-holeconductors T1 through T4.

In the high-frequency signal line 10 b having the structure describedabove, the insertion loss is significantly reduced for the same reasondescribed above in connection with the high-frequency signal line 10.

In the high-frequency signal line 10 b, damage to the dielectric sheets18 a and 18 b is prevented for the same reason described above inconnection with the high-frequency signal line 10.

The high-frequency signal line 10 b is easy to bend for the same reasondescribed above in connection with the high-frequency signal line 10.

It is possible to make the high-frequency signal line 10 b thinner forthe same reason described above in connection with the high-frequencysignal line 10.

Even when the high-frequency signal line 10 b is fixed to a metal objectsuch as the battery pack 206, a change in the characteristic impedanceof the signal lines 20 and 21 is prevented for the same reason describedabove in connection with the high-frequency signal line 10.

In the high-frequency signal line 10 b, there is another reason asfollows for the reduction in the insertion loss. When a current flows inthe signal line 20, lines of electric force occur intensively from theboth side portions in the y-direction of the signal line 20 to the mainground conductor 22 by edge effect. The intensive occurrence of lines ofelectric force from the both side portions in the y-direction of thesignal line 20 causes an intensive current flow in the both sideportions in the y-direction of the signal line 20. Accordingly, thecurrent passes in a small area of the signal line 20, and thus, thesignal line 20 is hard to pass a current.

In order to avoid this trouble, in the high-frequency signal line 10 b,the signal line 20 curves such that the central portion with respect tothe y-direction protrudes in the positive z-direction. Thus, the bothside portions in the y-direction of the signal line 20 are more distantfrom the main ground conductor 22 than the central portion of the signalline 20 with respect to the y-direction. Therefore, the intensiveoccurrence of lines of electric force from the both side portions in they-direction of the signal line 20 is prevented. Consequently, thecurrent passes in the whole area of the signal line 20, and the signalline 20 becomes easy to pass a current. Thus, in the high-frequencysignal line 10 b, the insertion loss is significantly reduced.

In the high-frequency signal line 10 b, there is still another reason asfollows for the reduction in the insertion loss. FIG. 15 indicates asectional view and a plan view of a non-curved signal line 20, and achart indicating current distribution. FIG. 16 indicates a sectionalview and a plan view of a curved signal line 20, and a chart indicatingcurrent distribution.

As illustrated in FIGS. 15 and 16, when a current flows in the signalline 20, a magnetic field is generated so as to go around the current.Then, due to electromagnetic induction, a countercurrent flowing in adirection opposite to the current is generated. In the central portionof the signal line 20 with respect to the y-direction, thecountercurrent flows on the both sides in the y-direction of thecurrent. In the positive side portion in the y-direction of the signalline 20, however, the countercurrent flows only on the negative side inthe y-direction of the current. Similarly, in the negative side portionin the y-direction of the signal line 20, the countercurrent flows onlyon the positive side in the y-direction of the current. Therefore, inthe signal line 20, the current value in the central portion of thesignal line 20 with respect to the y-direction is lower than the currentvalue in the both side portions of the signal line 20 in they-direction.

The width (size in the y-direction) of the signal line 20 illustrated inFIG. 16 is smaller than the width (size in the y-direction) of thesignal line illustrated in FIG. 15. Therefore, when the total currentflowing in the signal line 20 illustrated in FIG. 15 is equal to thetotal current flowing in the signal line 20 illustrated in FIG. 16, thecurrent flowing in the central portion, with respect to the y-direction,of the signal line 20 illustrated in FIG. 16 is greater than the currentflowing in the central portion, with respect to the y-direction, of thesignal line 20 illustrated in FIG. 15. Accordingly, a current flows inthe entire signal line 20 illustrated in FIG. 16, and the signal line 20illustrated in FIG. 16 as a whole is easy to pass a current. Thus, inthe high-frequency signal line 10 b, the insertion loss is reduced. Insum, an edge effect is seen in the side portions of the signal line 20.The edge effect hardly appears on the curved and narrow signal line 20,and the curved signal line 20 has a smaller insertion loss.

Third Modification

The structure of a high-frequency signal line 10 c according to a thirdmodification of a preferred embodiment of the present invention isdescribed with reference to the accompanying drawings. FIG. 17 is asectional view of the high-frequency signal line 10 c according to thethird modification cut at an opening 30. The appearance of thehigh-frequency signal line 10 c is as illustrated in FIG. 1.

The high-frequency signal line 10 c is different from the high-frequencysignal line 10 b in that via-hole conductors b1 through b4 and B1through B6 are used instead of the through hole conductors T1 throughT4.

In the high-frequency signal line 10 c having the structure describedabove, the insertion loss is significantly reduced for the same reasonas described above in connection with the high-frequency signal line 10b.

In the high-frequency signal line 10 c, damage to the dielectric sheets18 a and 18 b are prevented for the same reason described above inconnection with the high-frequency signal line 10 b.

The high-frequency signal line 10 c is easy to bend for the same reasondescribed above in connection with the high-frequency signal line 10 b.

It is possible to make the high-frequency signal line 10 c thinner forthe same reason described above in connection with the high-frequencysignal line 10 b.

Even when the high-frequency signal line 10 c is fixed to a metal objectsuch as the battery pack 206, a change in the characteristic impedanceof the signal lines 20 and 21 is prevented for the same reason describedabove in connection with the high-frequency signal line 10 b.

Fourth Modification

The structure of a high-frequency signal line 10 d according to a fourthmodification of a preferred embodiment of the present invention isdescribed with reference to the drawings. FIG. 18 is a sectional view ofthe high-frequency signal line 10 d according to the fourthmodification. The appearance of the high-frequency signal line 10 d isas illustrated in FIG. 1.

The high-frequency signal line 10 d is different from the high-frequencysignal line 10 in the positions of the main ground conductor 22 and theauxiliary ground conductor 24. More specifically, in the high-frequencysignal line 10 d, a dielectric sheet 18 d is placed on the positive sidein the z-direction of the dielectric sheet 18 a, and a dielectric sheet18 e is placed on the negative side in the z-direction of the dielectricsheet 18 c.

The main ground conductor 22 is provided on the lower surface of thedielectric sheet 18 d. The surface of the main ground conductor 22 incontact with the dielectric sheet 18 d has a greater surface roughnessthan the surface of the main ground conductor 22 out of contact with thedielectric sheet 18 d.

The auxiliary ground conductor 24 is provided on the upper surface ofthe dielectric sheet 18 e. The surface of the auxiliary ground conductor24 in contact with the dielectric sheet 18 e has a greater surfaceroughness than the surface of the auxiliary ground conductor 24 out ofcontact with the dielectric sheet 18 e.

In the high-frequency signal line 10 d having the structure describedabove, the insertion loss is significantly reduced for the same reasonas described above in connection with the high-frequency signal line 10.

In the high-frequency signal line 10 d, damage to the dielectric sheets18 a and 18 b is prevented for the same reason described above inconnection with the high-frequency signal line 10.

The high-frequency signal line 10 d is easy to bend for the same reasondescribed above in connection with the high-frequency signal line 10.

It is possible to make the high-frequency signal line 10 d thinner forthe same reason described above in connection with the high-frequencysignal line 10.

Even when the high-frequency signal line 10 d is fixed to a metal objectsuch as the battery pack 206, a change in the characteristic impedanceof the signal lines 20 and 21 is prevented for the same reason describedabove in connection with the high-frequency signal line 10.

In the high-frequency signal line 10 d, there is another reason asfollows for the reduction in the insertion loss. Specifically, when ahigh-frequency signal flows in the signal line 20, a current flowsintensively in the superficial portion of the signal line 20 by the skineffect. With respect to the signal line 20, the current flowsintensively especially near the surface facing the main ground conductor22 (that is, the surface out of contact with the dielectric sheet 18 b).Then, on the surface of the main ground conductor 22 facing the signalline 20 (that is, the surface of the main ground conductor 22 out ofcontact with the dielectric sheet 18 d), a countercurrent flows. Morespecifically, in the high-frequency signal line 10 d, the surfaceroughness of the surface of the main ground conductor 22 out of contactwith the dielectric sheet 18 d is smaller than that of the surface ofthe main ground conductor 22 in contact with the dielectric sheet 18 d.Therefore, the proportion of conductive material in an area from thesurface of the main ground conductor 22 out of contact with thedielectric sheet 18 d to a position at a specified depth is higher thanthe proportion of conductive material in an area from the surface of themain ground conductor 22 in contact with the dielectric sheet 18 d tothe position at the specified depth. Accordingly, the portion of themain ground conductor 22 near the surface out of contact with thedielectric sheet 18 d is easier to pass a current than the portion ofthe main ground conductor 22 near the surface in contact with thedielectric sheet 18 d. Consequently, in the high-frequency signal line10 d, the insertion loss is significantly reduced. The same phenomenonoccurs on the auxiliary ground conductor 24.

Other Preferred Embodiments

High-frequency signal lines according to the present invention are notlimited to the high-frequency signal lines 10 and 10 a through 10 d, andvarious changes are possible within the scope of the present invention.

It is possible to combine the structures of the high-frequency signallines 10 and 10 a through 10 d.

In the high-frequency signal lines 10 and 10 a through 10 d describedabove, the protective layers 14 and 15 are preferably formed by screenprinting, for example. However, the protective layers 14 and 15 may bepreferably formed by photolithography, for example.

The connectors 100 a and 100 b are not indispensable for thehigh-frequency signal lines 10 and 10 a through 10 d. In a case wherethe connectors 100 a and 100 b are not provided, the both ends of eachof the high-frequency signal lines 10 and 10 a through 10 d areconnected to circuit boards by solder or the like. It is also possiblethat only the connector 100 a is provided at only one end of each of thehigh-frequency signal lines 10 and 10 a through 10 d.

In the high-frequency signal lines 10 and 10 a through 10 d, theconnectors 100 a and 100 b are mounted on the top surface. However, theconnectors 100 a and 100 b may be mounted on the bottom surface. Also,the connector 100 a may be mounted on the top surface of each of thehigh-frequency signal lines 10 and 10 a through 10 d, and the connector100 b may be mounted on the bottom surface of each of the high-frequencysignal lines 10 and 10 a through 10 d.

Either one of the main ground conductor 22 and the auxiliary groundconductor 24 may be omitted from the high-frequency signal lines 10 and10 a through 10 d.

The auxiliary ground conductor 24 does not need to have openings.

Each of the high-frequency signal lines 10 and 10 a through 10 d may beused as a high-frequency signal line in an RF circuit board such as anantenna front-end module.

As thus far described, various preferred embodiments of the presentinvention and modifications thereof is useful in a high-frequency signalline, and preferred embodiments of the present invention andmodifications thereof has the advantage of preventing damage todielectric layers.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A high-frequency signal line comprising: adielectric body including a first dielectric layer and one or more otherdielectric layers laminated together in a lamination direction; a firstsignal line provided on a first main surface of the first dielectriclayer, the first main surface being a main surface of the firstdielectric layer located on one side of the first dielectric layer inthe lamination direction; a second signal line provided on a second mainsurface of the first dielectric layer to face the first signal line withthe first dielectric layer disposed therebetween, the second mainsurface being another main surface of the first dielectric layer locatedon another side of the first dielectric layer in the laminationdirection, and the second signal line being electrically connected tothe first signal line; a first ground conductor located on the one sideof the first dielectric layer in the lamination direction farther fromthe first main surface than the first signal line; and a second groundconductor located on the another side of the first dielectric layer inthe lamination direction farther from the second main surface than thesecond signal line; wherein a line width of the first signal line isgreater than a line width of the second signal line; and the firstsignal line is curved so as to protrude on one side in the laminationdirection.
 2. The high-frequency signal line according to claim 1,wherein a surface of the first signal line in contact with the firstdielectric layer has a greater surface roughness than a surface of thefirst signal line out of contact with the first dielectric layer; and asurface of the second signal line in contact with the first dielectriclayer has a greater surface roughness than a surface of the secondsignal line out of contact with the first dielectric layer.
 3. Thehigh-frequency signal line according to claim 1, wherein the secondground conductor includes openings aligned along the second signal line;and a distance in the lamination direction between the first signal lineand the first ground conductor is greater than a distance in thelamination direction between the second signal line and the secondground conductor.
 4. The high-frequency signal line according to claim1, wherein the dielectric body is flexible.